This invention relates generally to an analog electronic wristwatch of the type designed to optimize battery life. A very important technique to lower power consumption of an analog wristwatch is pulse width control in the pulses used to drive the step motor. It is desirable that the pulse width be no greater than is necessary to provide proper operation of the motor, that is, to have the rotor rotate 180.degree. when the pulse is applied. When a minimal pulse is applied, a problem arises to determine whether the rotor has actually rotated or not. Currently, a method is in use wherein rotation of the rotor is determined by detecting the current induced when the rotor oscillates transiently. But this method is intended to detect a very complicated phenomenon, that is, transient oscillation of the rotor, and various factors have a major influence on the detection of whether the rotor rotates or not. As a result, mass production difficulties and cost increase substantially.
A first principle in both the subject invention and in the prior art is to sense the magnitude of generated coil inductance from the difference in the stopping position of the rotor as a difference between rates of increase in currents. In the prior art, there is the problem that it is difficult to determine the rotation of the rotor because a detection resistor is used in series with the motor coil when a detection current is applied. The difference in the detection voltage across the detection resistor is small when comparing the rotated and non-rotated positions of the rotor. For this reason, a complicated system is required such that detection current is applied two times to the rotor in both the positive and negative directions to compare the detection voltage by using one voltage as a reference. Further, the resistance of the detection resistor must be precisely controlled because a scatter in the resistances of the detection resistors directly affects the scatter in the measured detection voltages. It is desirable to form the detection resistor as an integral part of the integrated circuit of the timeplace so as to meet the demands for a small, thin and inexpensive watch. However, it is impossible to form the detection resistor precisely as the scatter of detection resistance formed as part of an integrated circuit is large. Therefore, a detection resistor of high precision must be provided externally of the integrated circuit as an independently connected component, and this prevents the production of electronic watches which are small, thin and inexpensive and at the same time have pulse width optimization for power conservation.
What is needed is an electronic timepiece with pulse width optimization for driving the motor which does not require an externally connected precision resistance in the rotor position detection circuits.